As exploration and production of oil and gas has moved into deeper water, it has become increasingly important to reduce weight, lower costs, and improve reliability of water-depth sensitive systems such as risers and the like. The term riser generally describes the different types of discrete pipes that extend from the seabed toward the surface of the water. These include components such as drilling risers, production risers, workover risers, catenary risers, production tubing, choke and kill lines and mud return lines. Conventional risers are typically constructed of various metal alloys such as titanium or steel. More recently, however, the oil and gas industry has considered a variety of alternative riser materials and manufacturing techniques including the use of composite materials.
Composite materials offer a unique set of physical properties including high specific strength and stiffness, resistance to corrosion, high thermal insulation, improved dampening of vibrations, and excellent fatigue performance. By utilizing these and other inherent physical characteristics of composite materials, it is believed that composite riser may be used to lower system costs and increase reliability of risers used in deepwater applications. Although there has been a significant effort in the last decade to facilitate and to increase the general use of composites in offshore applications, the acceptance of composite materials by offshore operators continues to be a relatively slow and gradual process. Reasonably good progress has been made to expand the usage of composites for topside components such as vessels, piping and grating. Some advanced components such as high-pressure riser accumulator bottles have already been used successfully in the field. However, in view of the reduced weight, extended life span, lower cost and other enabling capabilities, composite risers are particularly appealing for deep water drilling and production operations.
Composite risers are generally constructed of a number of riser sections each having an outer composite material and an inner liner assembly. Typically, a thin tubular metal or elastomeric liner is coaxially secured to the metal connections at opposite ends to form the liner assembly. For a liner assembly comprising a metal liner, an elastomeric shear ply (usually rubber) is provided along the outer surface of the liner assembly, followed with a composite overwrap reinforcement to form the composite riser section. The composite riser section is then heated to cure the elastomeric shear ply and the composite overwrap. Additionally, an external elastomeric jacket and a layer of composite overwrap may be provided over the composite riser section and thermally cured to reduce external damage by providing impact protection and abrasion resistance to the composite riser section.
The liner assembly is necessary to prevent leakage due to the inherent cracking characteristics of the composite material. Typically, the matrix of the composite overwrap will develop micro cracks at pressures lower than those at which the reinforcing fibers of the composite structure will fail. Matrix micro cracking is due to the thermal stresses induced by the curing cycle and the mechanical stresses induced during the shop acceptance pressure test of the composite riser section during the manufacturing process. Thus, the liner assembly is essential in ensuring fluid tightness of a composite riser to prevent leakage under the condition of matrix micro cracking which is expected.
While elastomeric liners are generally acceptable for production composite risers, they are ill suited for use in composite drilling or workover risers. The likely possibility of damaging, namely cutting or tearing, elastomeric liners with the mechanical tools required for drilling and workover operations makes elastomeric liners less desirable for these types of operations. Accordingly, metal liners for composite drilling and workover risers are being considered. Metal liners also have applications in composite production risers as the metal liners may offer better long term resistance the corrosive production fluids than most elastomeric liners. In a typical composite riser having a metal liner, the metal liner is welded directly to the metal connection assembly at or near the metal-to-composite interface (MCI). Alternatively, the metal liner may be coaxially secured to the MCI through the use of a transition ring. The transition ring is secured at one end to the MCI and is welded at the other end to the metal liner and serves as a transition between the material of the liner and that of the MCI. A transition ring is generally used because the MCI and the connection assembly are generally constructed of a heavier tube stock than the relatively thin metal liner which serves primarily to keep the composite riser from leaking internal fluid. The transition ring is secured to the MCI either by welding or mechanically attaching it to the MCI. A mechanical attachment is preferred over welding when the transition ring and the MCI are formed of different materials.